There are ten different isomers of dimethyinaphthalene (DMN). Of these, nine of them can be grouped into three triads based on the relative ease of isomerization within a certain triad. Such an intra-triad isomerization can be done using a wide variety of solid acids as catalysts. This ease of isomerization within a triad is based on the fact that a methyl group on naphthalene shifts relatively easily from an alpha position to a beta position or vice versa on the same ring but does not shift easily from a beta position to another beta position on the same ring or from an alpha position to another alpha position. The three triad groups are as follows: 2,7-, 1,7- and 1,8-dimethylnaphthalene; 2,6-, 1,6- and 1,5-dimethylnaphthalene; and 1,4-, 1,3- and 2,3-dimethylnaphthalene. 1,2-dimethylnaphthalene is the tenth isomer and doesn't fit into any of the three triads. Although isomerization of dimethylnaphthalenes within these triad groups is relatively easy, isomerization from one triad group to another triad group is much more difficult. Since certain of the isomers of dimethylnaphthalene are much more valuable than others for use in plastics synthesis, investigators are continually making attempts to find ways of converting from less useful to more useful isomers. A particularly valuable isomer is 2,6-dimethylnaphthalene. Certain processes for synthesizing dimethylnaphthalenes result in high yields of 2,7- and 1,7-dimethylnaphthalenes. Conversion of 2,7- and 1,7-dimethylnaphthalenes into 2,6-dimethylnaphthalene has been accomplished using certain zeolites such as ZSM-5. However, such conversion has resulted in an excess of undesirable side products such as methylnaphthalenes, trimethylnaphthalenes and 1,4-, 1,3- and 2,3-dimethylnaphthalene via dealkylation, cracking and transalkylation. Usually, this acid-catalyzed isomerization is associated with catalyst deactivation as the reaction goes on, resulting in a short catalyst life.
It would be very useful to find an economical way to convert 2,7- and 1,7-dimethylnaphthalene which occur as abundant products in dimethylnaphthalene synthesis to 2,6-dimethylnaphthalene in a high yield.
Other investigators have found methods of converting the dimethylnaphthalene isomers, particularly 2,7-dimethynaphthalene to the most useful, and therefore most valuable isomer, 2,6-dimethylnaphthalene, but none of these conversion methods have been sufficiently simple and economical to warrant the general use of such methods.
U.S. Pat. No. 3,890,403 (Shimada et al.) discloses a method which can reportedly be used to obtain 2,6-dimethylnaphthalene from a dimethylnaphthalene mixture containing the various isomers of dimethylnaphthalene. The method involves (a) partially hydrogenating the dimethylnaphthalene mixture to obtain dimethyltetralins (DMT) with a hydrogenation catalyst such as nickel, platinum, palladium, rhodium, copper-chromium, iridium or ruthenium; (b) isomerizing the dimethyltetralins with a solid acid catalyst such as a zeolite catalyst so that the dimethyltetralin isomers in which the two methyl groups occur on the same ring can be converted to the dimethyltetralin isomers in which the two methyl groups occur on opposite rings and the amount of dimethyltetralin isomers in which the two methyl groups occur on opposite rings is brought near to thermodynamical equilibrium; (c) separating and collecting the dimethyltetralin isomers in which the two methyl group occur on opposite rings from the isomers in which the two methyl groups occur on the same ring; (d) dehydrogenating the collected DMT mixture to convert it into a DMN mixture; (e) separating and recovering 2,6-DMN from the recovered DMN mixture. Although this method obtains the desirable 2,6-DMN isomer from other DMN isomers, the method is quite time-consuming and expensive because it involves several quite separate and distinct steps.
U.S. Pat. No. 3,803,253 (Suld) discloses a process of hydroisomerization/dehydrogenation of a mixture of dimethylnaphthalenes, so that 2,6-dimethylnaphthalene can be obtained and isolated out from the reaction mixture. The other remaining products are then recycled and the process is repeated to obtain more 2,6-dimethylnaphthalene. The catalyst used for the hydroisomerization/dehydrogenation step is described as a combination of a calcium-containing faujasite and a hydrogenation/dehydrogenation catalyst component. The process step, with hydroisomerization and dehydrogenation performed simultaneously in the same reaction vessel in the presence of the described combination catalyst, simplifies the process but makes the overall efficiency and yield of the process quite low.
U.S. Pat. No. 3,928,482 (Hedge et al.), which is related to '253 discussed above, discloses a hydroisomerization process by which 2,6-DMT is obtained from a feed mixture which is rich in 2,7- or 1,7-DMT using an aluminosilicate zeolite containing polyvalent metal cations in exchange positions. This process is intended to be incorporated as an improvement to the method of '253 discussed above but does not overcome the basic lack of success of that process for obtaining 2,6-DMN in high yields in a cost-effective way.
An economical method of obtaining 2,6-DMN from other DMN isomers, especially isomers in the 2,7-DMN triad, with few steps and at relatively high yields is needed. The present inventors have found such a method.